Why would they need to arrest 150 million people? They'd let everyone in heavily Republican districts vote just fine, perhaps just a few random arrest at any precincts in Democratic areas. Their main focus would be urban areas, especially in blue states. And it wouldn't have to be everyone to get many, if not most people, to stay home. Early voting in your district? Great way to get ICE's arrests of people in line on the news before the big day, further driving down turnout. Filtering mail in ballots at the USPS not enough? Just happen to have some ICE agents drive by the drop boxes and oops, we saw an "illegal" voting, all these ballots are invalid, we'll be taking those. Local police try to step in (as if)? Insurrection Act, military deployed to all voting locations, ballots seized.
This shouldn't be hard to understand: there are any number of things an unfettered executive can do to turn the election that isn't simply cancelling them.
